Climate model simulations of the observed early-2000s hiatus of global warmingThe slowdown in the rate of global warming in the early 2000s is not evident in the multi-model ensemble average of traditional climate change projection simulations1. However, a number of individual ensemble members from that set of models successfully simulate the early-2000s hiatus when naturally-occurring climate variability involving the Interdecadal Pacific Oscillation (IPO) coincided, by chance, with the observed negative phase of the IPO that contributed to the early-2000s hiatus. If the recent methodology of initialized decadal climate prediction could have been applied in the mid-1990s using the Coupled Model Intercomparison Project Phase 5 multi-models, both the negative phase of the IPO in the early 2000s as well as the hiatus could have been simulated, with the multi-model average performing better than most of the individual models. The loss of predictive skill for six initial years before the mid-1990s points to the need for consistent hindcast skill to establish reliability of an operational decadal climate prediction system.

Two contrasting views of multidecadal climate variability in the twentieth centuryThe bulk of our knowledge about causes of twentieth century climate change comes from simulations using numerical models. In particular, these models seemingly reproduce the observed nonuniform global warming, with periods of faster warming in 1910–1940 and 1970–2000, and a pause in between. However, closer inspection reveals some differences between the observations and model simulations. Here we show that observed multidecadal variations of surface climate exhibited a coherent global-scale signal characterized by a pair of patterns, one of which evolved in sync with multidecadal swings of the global temperature, and the other in quadrature with them.In contrast, model simulations are dominated by the stationary—single pattern—forced signal somewhat reminiscent of the observed “in-sync” pattern most pronounced in the Pacific. While simulating well the amplitude of the largest-scale—Pacific and hemispheric—multidecadal variability in surface temperature, the model underestimates variability in the North Atlantic and atmospheric indices.

Pronounced differences between observed and CMIP5-simulated multidecadal climate variability in the twentieth century
Identification and dynamical attribution of multidecadal climate undulations to either variations in external forcings or to internal sources is one of the most important topics of modern climate science, especially in conjunction with the issue of human-induced global warming. Here we utilize ensembles of twentieth century climate simulations to isolate the forced signal and residual internal variability in a network of observed and modeled climate indices. The observed internal variability so estimated exhibits a pronounced multidecadal mode with a distinctive spatiotemporal signature, which is altogether absent in model simulations. This single mode explains a major fraction of model-data differences over the entire climate index network considered; it may reflect either biases in the models’ forced response or models’ lack of requisite internal dynamics, or a combination of both.

Plain Language Summary:Global and regional warming trends over the course of the twentieth century have been nonuniform, with decadal and longer periods of faster or slower warming, or even cooling. Here we show that state-of-the-art global models used to predict climate fail to adequately reproduce such multidecadal climate variations.In particular, the models underestimate the magnitude of the observed variability and misrepresent its spatial pattern. Therefore, our ability to interpret the observed climate change using these models is limited.

Distinct global warming rates tied to multiple ocean surface temperature changes
The globally averaged surface temperature has shown distinct multi-decadal fluctuations since 19001, 2, 3, 4, characterized by two weak slowdowns in the mid-twentieth century and early twenty-first century and two strong accelerations in the early and late twentieth century. While the recent global warming (GW) hiatus has been particularly ascribed to the eastern Pacific cooling5, 6, causes of the cooling in the mid-twentieth century and distinct intensity differences between the slowdowns and accelerations remain unclear7, 8. Here, our model experiments with multiple ocean sea surface temperature (SST) forcing reveal that, although the Pacific SSTs play essential roles in the GW rates, SST changes in other basins also exert vital influences. The mid-twentieth-century cooling results from the SST cooling in the tropical Pacific and Atlantic, which is partly offset by the Southern Ocean warming. During the recent hiatus, the tropical Pacific-induced strong cooling is largely compensated by warming effects of other oceans. In contrast, during the acceleration periods, ubiquitous SST warming across all the oceans acts jointly to exaggerate the GW. Multi-model simulations with separated radiative forcing suggest diverse causes of the SST changes in multiple oceans during the GW acceleration and slowdown periods. Our results highlight the importance of multiple oceans on the multi-decadal GW rates.

A long-standing mystery is that, despite the persistently increased greenhouse gases emissions throughout the twentieth and early twenty-first centuries, the globally-averaged surface temperature has shown distinct multi-decadal fluctuations since 1900, including two weak global warming slowdowns in the mid-twentieth century and early twenty-first century and two strong global warming accelerations in the early and late twentieth century. The multi-decadal global warming rate changes are primarily attributed to multiple ocean surface temperature changes, according to research by Institute of Atmospheric Physics and Australian Bureau of Meteorology. It is the net impact of multiple ocean surface temperature changes, rather than a single ocean basin change, that plays a main driver for the multi-decadal global warming accelerations and slowdowns. Understanding and quantifying the respective role of individual ocean basin in the multi-decadal global warming accelerations and slowdowns, under the forcing of the sustained increase in atmospheric greenhouse gas concentrations, could help achieve a more accurate estimate of the future global warming rate to better meet the global warming target of the Paris Conference reached in December 2015–no more than 1.5ºC above pre-industrial levels by 2100.

The new finding of the importance of multiple ocean surface temperature changes to the multi-decadal global warming accelerations and slowdowns is supported by a set of computer modeling experiments, in which observed sea surface temperature changes are specified in individual ocean basins, separately. The results are published in “Distinct global warming rates tied to multiple ocean surface temperature changes”, in the June 12 online issue of Nature Climate Change.

“The results elucidate the relative contributions of individual ocean surface temperature changes to the multi-decadal global warming rate changes, and could help improve our understanding of global warming fluctuations under steadily increased emissions of atmospheric greenhouse gases.” says Jing-Jia Luo, the corresponding author of the study and climate scientist at the Bureau of Meteorology in Australia. “It reveals a fact that we need to explore climate change in a more global perspective. This could stimulate an integrated strategy and coordinated effort toward understanding the causes of regional ocean changes.”

“Our study provides a novel perspective for understanding and projecting individual ocean basin’s impacts on global warming,” explains co-author Dr. Shuai-Lei Yao from CAS Institute of Atmospheric Physics. “While the tropical Pacific was generally regarded as a key contributor to the multi-decadal global warming rate changes, other ocean basins, including the Indian Ocean, the Atlantic and the Southern Ocean, also exert important effects. “